This invention relates to a polyolefin composition containing an inorganic filler. More particularly, it is concerned with an improved polyolefin composition which is prepared by incorporating an inorganic filler into a modified polyolefin obtained by adding an unsaturated carboxylic acid or an anhydride thereof into a polyolefin, or a mixture of such a modified polyolefin and an unmodified polyolefin, and which further contains a nucleating agent or a combination of a nucleating agent and a heat deterioration inhibitor. The improved polyolefin composition does not undergo any substantial deterioration by heat or substantial reduction of its mechanical strength during a molding operation.
It is known to incorporate an inorganic filler into a polyolefin, such as polyethylene and polypropylene, in order to improve its mechanical strength, dimensional stability and heat resistance. Various methods have been proposed for modifying a polyolefin, or treating an inorganic filler, in order to improve the adhesion between the polyolefin and the inorganic filler. For example, it is known to graft a poly-olefin with an unsaturated carboxylic acid or an anhydride thereof, such as acrylic acid, maleic acid and endo-cis-bicyclo[2,2,1]-5-heptene-2,3-dicarboxylic anhydride (Japanese Patent Publication Nos. 36421/1970, 4822/1974, 7334/1974, 15467/1975 and 49029/1974, and Japanese Patent Application Laid-Open Specification No. 110754/1977), or to treat the surfaces of glass fibers with an organic silane compound, a vinyl acetate resin, an ethylene-methyl acrylate copolymer, an ethylenevinyl acetate copolymer, or the like (Japanese Patent Publication Nos. 36421/1970, 7334/1974 and 49029/1974, and Japanese Patent Application Laid-Open Specification No. 110754/1977). Polyolefin resins reinforced with carbon or glass fibers have a very high mechanical strength, and are regarded as useful engineering plastics for a variety of applications.
The mechanical strength of any such polyolefin resin reinforced with an inorganic filler is, however, undesirably lowered when it is molded into a product having a large wall thickness. This reduction in mechanical strength is apparently due to the fact that the deterioration of the resin by heat occurs during its prolonged exposure to a high temperature. In extrusion, transfer or injection molding of a product having a large wall thickness of, say, at least 100 mm from the resin requires a cooling time which is as long as at least half an hour, and that the rapid growth of crystal grains of the polyolefin in the vicinity of its crystallization temperature lowers the affinity of polyolefin grains for one another and for the reinforcing filler, resulting in the low mechanical strength of the molded product.
In accordance with this invention it has been discovered that the addition of a heat deterioration inhibitor (i.e., oxidative inhibitor or stabilizer) and a nucleating agent can prevent any such reduction in the mechanical strength of a molded polyolefin product obtained by slow cooling.